There is conventionally known such a cartridge that is mounted in an image forming apparatus and that has a photosensitive body cartridge and a developer cartridge. The photosensitive body cartridge is provided with a photosensitive body, while the developer cartridge accommodates developer therein. The photosensitive body cartridge and the developer cartridge can be changed with new cartridges individually from each other.
The conventional developer cartridge has a sensor gear that rotates irreversibly when receiving input of a drive force from the main body of the image forming apparatus. The sensor gear is provided with a contact projection. An actuator is provided in the main body of the image forming apparatus. Depending on whether the contact projection contacts the actuator, the image forming apparatus can detect whether the developer cartridge is in a new state.